Case 1: shrink on the left
Click the word: project on the left to display the file list, or click the small icon in the lower left corner to pop up and select: project.
Situation 2: There is no contracted project on the left,
Method 1: Click the layer menu in the lower left corner and select Project
， 
Method 2: Shortcut key: Alt+F1, select Project View in the pop-up window
Method 3: Double-click Shift to open search anywhere , enter: Project
The ultimate trick:
The three methods in case 2 do not work, restart Android Studio, and try again. If not, restart several times.